a. When all boxes are installed, installation of conductors can begin. A common problem that arises is how to determine the number of conductors that can be installed in a box. This problem is easily solved by consulting the NEC Handbook. A sample NEC table is shown in Table 1. This table shows the number of conductors that can be installed in a box, based on the American Wire Gauge (AWG) classification of the wire.

b. Other factors also affect the number of conductors that can be mounted in a box. Different size conductors require different amounts of free space. This free space is measured in cubic inches. Table 2 is a sample NEC table showing the amount of free space required per conductor, based on the largest size conductor in the box. The number of conductors permitted is reduced by one for each fixture stud, fixture extension, internal connector, or clamp (Figure 20). Reduce the number of conductors by two for each switch or receptacle. No allowance is required for a connector with a connecting device outside of the box. Locknuts and bushings do not count.

c. Quite often, especially in a junction box, more than one size of conductor will terminate in the box. If no devices are present, calculating how many conductors can fit into the box is accomplished by consulting the NEC Handbook. Under normal conditions, there will be some type of cable or internal connector or clamp and possibly a device in the box. Two conductors are subtracted for each device. The following steps show how to determine which size of conductor will be subtracted when various sizes are present in the box:

Step 1. Determine the cubic-inch capacity of the box being used by consulting the appropriate table in the NEC. For this course, use the sample NEC table in Table 2.

Step 2. Determine the amount of free space needed for the largest conductor entering the box.

Step 3. Total the number of the largest conductors. Add one for all internal clamps, and add two for each switch or receptacle.

Step 4. Multiply the number from step 3 by the free space needed for the largest conductor.

Step 5. Total all remaining conductors (by size), and multiply each individual total by the free space required for its size.

Step 6. Add the answer from step 4 to the answer from step 5.

Step 7. Subtract the answer to step 6 from the answer to step 1.

If the final answer to step 7 is a positive number, the conductors and device(s) will fit. If the answer is a negative number, a larger box is needed. See the example shown in Figure 21.

a. Grounding conductors are counted differently. Regardless of how many grounding conductors are coming into a box, the total will be counted as one conductor (Figure 22). The largest grounding conductor determines how much free space is required. For example, three 14-AWG and one 12-AWG grounding conductors will be counted as one 12-AWG conductor.

b. Conductors that terminate inside the box without being connected to a device count as one each. Conductors that pass through the box also count as one each. Conductors that originate in the box and never leave the box do not count. An example of this would be fixture wires. If more free space is needed, extension rings and/or additional boxes can be added to provide the additional space required.